1. Field of the Invention
The present invention relates to a method of fabricating an ultrasonic coupler which is interposed between an ultrasonic probe and an object to be observed with the use of an ultrasonic diagnostic system.
2. Description of the Related Art
There has been generally used an ultrasonic diagnostic system for displaying a tomographic image of the human body utilizing ultrasounds. Now, referring to the drawings, there will be explained a case where such an ultrasonic diagnostic system is used to observe diagnostic portions of the human body, for example, a thyroid gland, a carotid artery, and a mammary gland, which all exist near the body surface.
FIG. 7 is a view schematically showing a state in which an ultrasonic probe for transmission and reception of ultrasounds directly contacts with the body surface. FIG. 8 is a view schematically showing a state wherein an ultrasonic coupler, which is sufficiently conductive in ultrasound, is placed between an ultrasonic probe and the body surface.
As shown in FIG. 7, in a case where an ultrasonic probe 10 directly contacts the body surface 12, a convergent point of an ultrasonic beam 14 is given by a position 16 which is placed at a distance from the body surface 12. Consequently, when a diagnostic portion 18, which exists near the body surface 12, is observed, a resolving power will degrade. Thus, in such a case, in order to contribute to an enhancement of the resolving power, it is so designed, as shown in FIG. 8, that an ultrasonic coupler 20 is placed between the ultrasonic probe 10 and the body surface 12, so that the ultrasonic beam 14 can be converged on the neighborhood of the diagnostic portion 18.
Such an ultrasonic coupler is made of a non-hydrous gel substance consisting of a urethane rubber, a silicone rubber or the like, and alternatively, a polymer hydrous gel substance such as a polyvinyl alcohol (PVA), a polyvinyl pyrrolidone (PVP), a polyethylene oxide (PEO) and the like.
In case of the use of the non-hydrous gel substance for the ultrasonic coupler, the ultrasounds transmitted from an ultrasonic probe will be attenuated during passing through the ultrasonic coupler made of the non-hydrous gel substance, since the attenuation of the ultrasounds is generally large (approximately, 2 dB/cm/MHz) within the non-hydrous gel substance. Consequently, the ultrasounds hardly enter the object to be monitored. Similarly, the ultrasounds reflected by the inner observed object will be also attenuated during passing through the ultrasonic coupler. Thus, it will be a cause of deterioration of the ultrasound signal to noise ratio (S/N).
On the other hand, in case of the use of the ultrasonic coupler made of the polymer hydrous gel substance, it is possible to implement a high S/N, since the attenuation of the ultrasounds is extremely small (approximately, 0.1 db/cm/MHz) within the polymer hydrous gel substance. However, if such an ultrasonic coupler is left in an atmosphere, water evaporates and it will spoil the function of the ultrasonic coupler. Therefore, it is necessary to preserve the ultrasonic coupler in a sealed vessel when it is not used. Further, such an ultrasonic coupler easily slips and it is difficult to hold by hand. In view of the foregoing, there is used an ultrasonic coupler 24, as shown in FIG. 9, on which connection members 22 for connecting the coupler with the ultrasonic probe 10 are fixed. An aqueous solution containing a PVA among the polymer hydrous gel substance has such a characteristic that when the aqueous solution is frozen below the freezing point and defrosted at the room temperature, a bridging between molecules progresses thereby, gelling. A repetition of freezing and defrosting advances an additional bridging (refer to Japanese Patent Publication No. Sho. 47-12854). The gelling substance (PVA gel) has an excellent mechanical characteristic in points of a high tearing strength, and a full elasticity. Thus, the gelling substance is suitable for the ultrasonic coupler.
Next , referring to the drawings, there will be explained a method of fabricating an ultrasonic coupler, which is made of an aqueous solution containing the PVA, among methods of fabricating an ultrasonic coupler.
FIGS. 10A-10F are sectional views showing steps of fabricating an ultrasonic coupler.
First, as shown in FIG. 10A, a specified configuration of die 30 is prepared. Next, as shown in FIG. 10B, an aqueous solution 34 containing the PVA is poured from an inlet 32 of the die 30 into the die 30. After pouring the aqueous solution 34, it is frozen below the freezing point. The aqueous solution 34 will increase in volume about 10% when frozen. Consequently, as shown in FIG. 10C, for example, it will happen that a parting segment 36 of the die 30 is destroyed, or expanded portions are formed. As a result, as shown in FIG. 10D, a gel 38 having a projection 40 is made up. The steps of FIGS. 10C and 10D are repeatedly performed. Thereafter, as shown in FIG. 10E, the gel 38 is separated from the die. Finally, as shown in FIG. 10F, the expanded portion or the projection 40 is excised so as to reshape into a specified configuration of ultrasonic coupler 44.
As described above, since the aqueous solution containing the PVA will increase in volume about 10% when frozen, according to the above described fabrication method, there will be formed the projections due to gelling. Thus, it is necessary to excise or remove the projections and it will be troublesome. Further, a surface, which is formed by removal of projections, will be poor in smoothness. If such a surface is in contact with a body surface or an ultrasonic probe, it will be a cause of a large attenuation of the ultrasounds.